Assessment Criteria for Pop-Ins Generated in Welded Joints Subjected to Fracture Toughness Testing

A pop-in (a brittle crack arrest phenomenon) sometimes occurs in a three-point bending fracture toughness test of a welded joint. The current standards may consider a pop-in to be a significant fracture and underestimate fracture toughness. In this case, the allowable defect size in a welded structure becomes very small, and many defects need to be repaired, resulting in high maintenance costs. Therefore, a reasonable acceptance judgment method for the pop-in is practically important. Herein we proposed an assessment method to determine whether or not it is a significantly detrimental pop-in that affects the integrity of a structure. The method was designed based on the cohesive zone model and validated on its ability to reproduce experimental results. The difference in loading modes between bending during the fracture toughness test and tension of the structure and in the crack initiation conditions for both load modes was determined by comparing the Weibull stress, which is a characteristic of the material. All the main fracture surfaces of the numerical model were cohesive and had conservatively low toughness. No pop-in occurred under tensile loading under a Weibull stress condition of 13.1 % load drop, whereas a pop-in occurred under bending loading. However, under a load drop of 10.6 %, a pop-in occurred even under tensile loading. Therefore, it was shown that the load drop could be relaxed to 10 % or more, although the load drop is considered insignificant at 5 % or more according to the current standards.